1. Field of the Invention
The present invention relates generally to rodless pumps and in particular to a rodless pump comprising a gas spring.
2. Description of the Prior Art
Presently, low pressure, non-flowing oil wells account for over 90% of the oil wells in the United States. There are various means available for pumping these non-flowing oil wells. The most common of these pump means is the sucker rod type subsurface pump. Other types of pumps include electrical and hydraulic actuated subsurface pumps. One problem which is common to each of these subsurface pumps is that they require a separate energy transmission path for supplying the actuating energy to the pump.
Although sucker rod type pumps are not the most energy efficient, they are probably the most reliable. However, sucker rod failures are still a major problem, as studies have shown that a sucker rod fails an average of once every two years. These failures result in significant repair and maintenance costs.
There have been several attempts to provide a rodless subsurface pump system which does not require a separate energy transmission path for activating the pump. This type of pump system typically includes a surface unit which is connected to the subsurface pump by a single fluid conduit. The surface unit activates the subsurface pump by applying pressure to the fluid in the conduit to compress a spring means in the pump and displace a slidable piston to draw fluid from the well into a pump chamber. When the surface unit releases the fluid pressure, the spring means of the subsurface pump will displace the piston and lift the fluid in the pump chamber into the fluid conduit. Such systems are disclosed in U.S. Pat. Nos. 2,058,455; 2,123,139; 2,126,880 and 2,508,609.
However, these pressure activated subsurface pump systems have some inherent problems. When fluid pressure is applied to the fluid conduit, the actual energy applied to the system is much greater than the energy supplied to the subsurface pump. Since thousands of feet typically separate the surface unit and the subsurface pump, considerable work is done compressing the fluid in the conduit, ballooning the conduit, and moving fluid to compress the subsurface pump spring. In these systems, considerably more energy is consumed in compression and ballooning than is used to lift fluid. Thus, these systems are energy inefficient.
There are also several problems associated with these subsurface pumps. Typically, it has been desirous to provide a subsurface pump having a relatively long stroke length such that more fluid could be produced for a given amount of energy input. However, early subsurface pumps utilized strong helical compression springs as a means for lifting the fluid into the fluid conduit. These springs severely limited the maximum stroke length which could be attained.
Other subsurface pumps, such as the one disclosed in U.S. Pat. No. 4,013,385, utilize an inert gas pressurized chamber which functions as the spring means. When pressure is applied to the fluid conduit, a piston will compress the gas within the chamber and, when the fluid pressure is relieved, the gas will expand to lift fluid into the conduit. However, in this type of subsurface pump, it is difficult to maintain an effective seal between the gas chamber and the associated fluid.